1. Field of the Invention
This invention relates to biological inhibitor abatement in the context of preparing agricultural biomass. Agricultural biomass, such as corn fiber or corn stalks, can be used as an abundant and low-cost substrate for bioethanol production [Bothast, R. J. and Saha, B. C. (1997) 44:261–286] or production of other value-added products. Sugars contained in the lignocellulosic matrix are first released by hydrolysis, and then fermented to ethanol or other value-added chemicals by microorganisms. Biomass is made up of xylan and celluose polymers that must be pretreated to release sugars, prior to fermentation. Several pretreatment methods have been proposed [Hsu, T. (1996) Pretreatment of biomass. In: Handbook on Bioethanol: Production and Utilization. Wyman C. E. (Ed). Washington, D. C.: Taylor and Francis]. Most use conditions that create inhibitors simultaneous-with the production of sugars. For example, the resultant hydrolysate from dilute acid pretreatment comprises a complex mixture of components, in which more than 35 potentially toxic compounds have been identified [Luo, C. et al. (2001) Biomass Bioenergy 22:125–138]. These compounds can be divided into three main groups: organic acids (acetic, formic and levulinic acids), furan derivatives (furfural and 5-hydroxymethylfurfural), and phenolic compounds. They derive from sugar degradation or are directly released from the lignocellulose polymer [Palmqvist, E. and Hahn-Hägerdal, B. (2000) Biores. Technol. 74:25–33]. They affect the overall cell physiology and often result in decreased viability, ethanol yields and productivity (Palmqvist and Hahn-Hägerdal, supra), and possibly total failure of the fermentation process. Although ethanologenic microorganisms may degrade some of these compounds, the toxicity of the hydrolysate will likely persist as a result of the aggregate effect of the remaining compounds [Zaldivar, J. et al. (2001) Appl. Microbiol. Biotechnol. 56:17–34].
2. Description of the Prior Art
Several approaches have been suggested for overcoming the negative effect of inhibitors in hydrolysates, including: the reduction of their formation by adjusting sugar extraction conditions; the use of tolerant ethanologenic strains; and hydrolysate detoxification (Zaldivar J. et al., supra).
The most established methods for hydrolysate detoxification include the addition of ion exchange resins [Nilvebrant, N. O. (2001) Appl. Biochem. Biotechnol. 91/93:35–49], addition of active charcoal [Gong, C. S. et al. (1993) Appl. Biochem. Biotechnol. 39/40:83–88], enzymatic detoxification using laccase [Jönsson, L. J. et al. (1998) Appl. Microbiol. Biotechnol. 49:691–697] and overliming [Martinez, A. et al. (2001) Biotechnol. Prog. 17:287–293]. Notwithstanding their benefits, these methods have certain limitations [Zaldivar, J. et al., (1999) Biotechnol. Bioeng. 65:24–33]; namely, producing waste and/or adding significantly to the cost of production.
In contrast, microbiological abatement represents an improvement compared to the aforementioned approaches to detoxification. Biological abatement generates little waste, and it may be performed directly in the fermentation vessel prior to fermentation. However, to date, few biological treatment processes have been studied. Schneider [Enzyme Microb. Technol. 19:94–98 (1996)] proposed a treatment using a S. cerevisiae mutant for acetic acid removal from acid hydrolysate without altering sugars. Treatment with soft-rot fungus Trichoderma reesei that degrade inhibitors in a hemicellulase hydrolysate has also been reported [Palmqvist, E. et al. (1997) Enzyme Microb. Technol. 20:286–293].